Freerunning and Entrained Circadian Rhythms

Abstract

Orcadian rhythms “freerun” in constant conditions like self-sustaining oscillations, and they can be synchronized (entrained) by periodic factors in the environment, the Zeitgebers. The period τ of the freerunning rhythm depends on the species, on the individual and its physiological state, on environmental conditions, and on the experimental history. Under conditions of entrainment, the rhythm keeps a distinct phase-relationship to the Zeitgeber. The record of locomotor activity of a pig-tailed macaque, Macaca nemestrina, reproduced in Figure 1, illustrates a few of the major principles. When exposed to a light-dark cycle (LD), the monkey is active during L, with onset of activity occurring shortly before light-on; in constant conditions (LL), the period is shorter than 24 hr and, on the average, somewhat shorter in 0.03 lux than in 0.1 lux; there is day-to-day variability of intervals between the onsets of activity around the mean τ that in itself changes slightly over time (especially in 0.03 lux); reentrainment by the Zeitgeber after Day 78 is accomplished by a series of delay transients. A more detailed discussion of those phenomena follows below.